Near Washington, D.C., Joseph Grimmer and several neo-Nazi Germans chase a launch craft through river fog until the pursued boat runs up a ramp and crashes into a warehouse. Two men dressed in Alice in Wonderland costumes escape over a fence, but Grimmer’s men kill two others. Elsewhere, MacArthur “Mac” Stern and his Vice Squad partner, Drummond, are called to an apartment building to quell two noisy lovers. The cops threaten to arrest the man unless he goes to the store and buys condoms. When Mac returns to police headquarters, another detective informs him that Captain Del Doggett wants him to return to the Homicide Squad downtown, where he worked before being exiled for misbehavior. Meanwhile, as Ellis Fielding paints pictures at a Benedictine monastery, a monk-psychologist tells him this therapy is over, because he has been called back to police duty. The next morning, Mac sleeps in front of the downtown precinct in his vintage Ford 'Woody’ station wagon, which is filled with his cat and his possessions, because his apartment caught fire. A policeman awakens Mac with the news he is wanted at a crime scene in the marshes outside the city. When Mac arrives, Capt. Del Doggett welcomes him back to Homicide and shows him two corpses in Alice in Wonderland costumes. Ellis Fielding is also there, vomiting from the gruesome sight. The captain introduces Ellis as a forensics expert with an unusually “analytical mind.” As soon as Ellis finishes throwing up, he reconstructs the murders with deductive reasoning. One of the victims, dressed as the “Cheshire Cat,” is German, and one of the men who escaped was obese and dressed as the “Queen of Hearts.” Capt. Doggett informs Mac that Ellis is his new partner. However, as they drive back to headquarters in Mac’s Woody, it is clear the two men are incompatible. Mac is messy, and Ellis is neat. Mac smokes cigarettes and has a cat, while Ellis is allergic to both. Ellis explains that he has been on sick leave for two years. He generally annoys people, and assumes the identities of fictional characters when he is under pressure. At headquarters, a desk officer tells Mac the crashed boat belongs to Harry “The Hippo” Gutterman, a fat pornographer who runs a sadism and masochism (S&M) club in Washington’s industrial district. Surmising that Harry was the “Queen of Hearts,” Mac wants to find him before the killers do. When Mac and Ellis enter Guttman’s S&M club, Mac gets into a fight with security men. Ellis assumes the voices of several characters, including the “Cowardly Lion” from The Wizard of Oz, and distracts the thugs until one smashes him over the head with a bottle. Once Mac identifies himself, Gutterman demands police protection. On the way to headquarters, Gutterman explains that several Germans want to kill him because he saw a pornographic movie with Adolf Hitler and two men, along with some “historical stuff.” Everyone who has seen the film is dead, except for him and Steckler, a German who stole the film and is trying to sell it. Muller, the murdered “Cheshire Cat,” was the middleman who introduced Steckler to Gutterman. Suddenly, Ellis sees a car behind them, with Joseph Grimmer ready to fire an anti-tank rocket. Mac slams on the brakes to avoid the missile, and it blows up several parked vehicles. After a gunfight, the bad guys drive off, but Mac tells Ellis to get behind the wheel and give chase while he pours gunpowder into an antique blunderbuss. Frightened, Ellis impersonates several Star Trek characters as he drives. The neo-Nazis escape, but not before Mac’s Woody is destroyed. At headquarters, as Mac complains to Capt. Doggett about his new partner, the captain explains that Ellis is his nephew. While working undercover, Ellis was tortured by Colombian drug dealers and developed a multiple personality disorder, which required institutionalization at a Benedictine monastery. Doggett informs Mac that the missing Steckler was the personal secretary of Kort Von Metz, an ultra-conservative politician recently elected as West German Chancellor, despite rumors that he was one of Hitler’s favorite officers during World War II. Having destroyed Mac’s “home” in the car chase, Ellis invites him to stay in a spare bedroom in his apartment. Meanwhile, in an American television studio, an interviewer asks Von Metz about his rumored association with Hitler, but Von Metz denies ever meeting the Nazi leader. Grimmer approaches Von Metz’s associate, Gerber, who is shocked by Grimmer’s trail of murder victims and orders him to bring Steckler back alive. Grimmer taunts Gerber for allowing Steckler to steal the film in the first place. When Mac arrives at Ellis’s apartment, he is surprised to find that everything is painted white or light gray. Ellis explains that he needs as little stimuli as possible, in order to keep his personalities in check. He plays nature tapes of surf lapping on a beach. That night, Mac awakens hearing Ellis babbling in several voices, but in the morning, everything seems fine. Driving to the West German embassy, Mac and Ellis notice a man and woman following them. Inside the embassy, FBI (Federal Bureau of Investigation) agent Bob Smiley pulls the cops aside and tells Mac the multiple homicide case should not interfere with “national security priorities.” Smiley has a writ being drafted for Gutterman’s arrest and will be at police headquarters in an hour to take custody. Mac and Ellis immediately return to headquarters and interrogate Gutterman, threatening to telephone television media if he does not confess Steckler’s whereabouts. Gutterman confesses he will meet Steckler tomorrow at a New York City bathhouse. Mac and Ellis decide to drive to New York City, but they need Gutterman to identify Steckler. Seeing Smiley and his agents approaching outside, Mac and Ellis commandeer a police van and drive to the train station, followed by FBI agents in one car and the man and young woman in another. At the station, Mac and Ellis elude the federal agents by taking Gutterman on the wrong train, but the young woman and man climb aboard with them. The woman identifies herself as Israeli Mossad agent Rebekka “Reva” Lowengruen, and tells Mac she is looking for a stolen film that may show Von Metz was in Hitler’s suicide bunker. Joseph Grimmer and his neo-Nazis pursue the train in helicopters and force it to stop on a railroad bridge. Mac and Ellis, with Gutterman in tow, leap into the water below to avoid being machine-gunned and hide in a forest until morning. Ellis begs Mac to return him to the monastery, but Mac thinks he can be a functional policeman again. In the morning, they rent a car and drive into New York City, where Gutterman leads them to the Hadrian bathhouse, which he owns. Ellis tells his partner that Riva may also be there because he told her where they were going. She wants to show the film that night when Von Metz addresses the International Conference of Christians and Jews. Gutterman greets Steckler, but Joseph Grimmer’s men have already arrived and shoot the German. Before dying, Steckler tells Ellis where he hid the film. When Grimmer’s men capture the two cops and the fat pornographer, Ellis agrees to lead them to the film. To alert Riva nearby, Gutterman yells that the film is at Grand Central Station. Grimmer shoots and wounds him, but Gutterman falls over a railing to freedom. The Germans take the cops into the subway and lead them toward a tunnel, but Mac escapes on a train and rides to the next station. To get to Grand Central Station, he commandeers a bus and orders the driver to crash through traffic. At the train terminal, Mac finds Ellis in the basement and saves him from being killed, although Ellis is shot in the leg. The Germans rush to the public lockers to get the film, but Ellis tells Mac he gave them the wrong locker number. Mac hurries to the lockers and gets the film can, but after he escapes into the heating duct system above the station lobby, Grimmer gets the jump on him. Rather than surrender the film, Mac throws the canister through a glass photo display. It lands below on the floor of the lobby, where Riva and another Israeli agent retrieve it. Grimmer shoots Mac in the leg, but Ellis arrives in time to kill the neo-Nazi and send him crashing through the photo display. That night, as Von Metz speaks at the conference, Israelis expose him by showing a film of him as a young Nazi officer, delivering the coup de grâce to Hitler. Gutterman, Ellis and Mac end up at the same hospital, where Ellis entertains children with his characters. Mac and Ellis decide to partner up again as soon they can walk.